1. Field of the Invention
The present invention pertains to a pressure support apparatus and method for controlling a pressure support therapy administered to a patient, and, in particular, to a controlling a proportional assist ventilation or a proportional positive airway pressure mode of ventilating a patient by varying at least one parameter associated with one of these ventilation techniques according to a parameter variation function.
2. Description of the Related Art
It known to administer a therapeutic treatment to a patient in order to treat a breathing disorder or to provide a patient with life support ventilation via a ventilator or pressure support device. There are a variety of techniques for providing the flow of breathing gas to the patient, commonly referred to as modes of ventilation. Two such conventional modes of ventilation are proportional assist ventilation (PAV(copyright)) and proportional positive airway pressure (PPAP) ventilation.
PAV is taught by U.S. Pat. Nos. 5,044,362 and 5,107,830 both to Younes et al., the contents of which are incorporated herein by reference. According to this technique, the pressure delivered by the ventilator to the patient increases in direct proportion to patient breathing effort, so that the greater the patient effort, the greater the pressure of breathing delivered by the ventilator. More specifically, the pressure of gas delivered by the ventilator to the patient is determined based on a product of a first gain and a signal indicative of a rate of flow of breathing gas provided to such a patient in combination with a product of a second gain and signal indicative of a volume of breathing gas provided to the patient. This relationship is summarized as follows:
Ppatient=FA*{dot over (V)}+VA*V,xe2x80x83xe2x80x83(1)
where Ppatient is the pressure at the patient interface device, such as the patient""s nasal mask, nasal/oral mask, nasal cannula, or trachea tube, FA is a flow assist multiplier, which is a fraction of respiratory resistance, {dot over (V)} is the instantaneous flow of gas to the patient, VA is a volume assist multiplier, which is a fraction of respiratory elastance, and V is the instantaneous volume of gas delivered to the patient.
In a slight variant of the above-described PAV mode, pressure support is provided to a patient according to the following equation:
Ppatient=% Set (FA*{dot over (V)}+VA*V)+EPAP,xe2x80x83xe2x80x83(2)
where %Set is a common multiplier or scaling factor applied to both FA*{dot over (V)} and VA*V, and EPAP is the expiratory positive airway pressure, which is a minimum baseline pressure applied to the patient during the expiratory phase of his or her breathing cycle.
In order to provide a patient with PAV as a mode of pressure support therapy or as a mode of ventilation, the caregiver sets the following parameters of equations (1) or (2): %Set, FA, VA, and EPAP. This is done either based on data garnered from patient observation and caregiver experience and expertise, data gathered from measurements of various physiological parameters of the patient, such as respiratory elastance and resistance, or based on a predetermined values that are generally believed to be appropriate for a given group of patients. Once the patient begins receiving PAV, he or she continues to receive PAV at the initially set parameters until the caregiver alters the parameters. For example, as the patient""s physical condition improves he or she may need less assistance from the ventilator, so the caregiver may reduce the %Set level to reduce the assistance to breathing provided by the ventilator. It is also known to set one of more of the parameters, %Set, FA, VA, and EPAP, automatically, to modify these settings automatically, or both.
PPAP ventilation is taught by U.S. Pat. Nos. 5,535,738 and 5,794,615 both to Estes et al., the contents of which are incorporated herein by reference. According to this technique, the pressure of gas provided to the patient is a function of the patient flow rate. This relationship can be summarized as follows:
Ppatient=Pbase+Gain*{dot over (V)},xe2x80x83xe2x80x83(3)
where Ppatient is the pressure delivered to the patient interface device, Pbase is the base line pressure (greater than or equal to zero and conceptually equal to EPAP), {dot over (V)} is the instantaneous flow of gas to the patient, which can be measured directly via a flow transducer or estimated from a flow transducer measurement and a leak component determination, and Gain is a constant used to augment the pressure delivered to the patient based on the flow rate.
In a variation of the PPAP ventilation technique, the pressure delivered to the patient is described according to the following functions:                                           P                          patient              ⁡                              (                insp                )                                              =                                    P              base                        +                                          Gain                insp                            *              Flow                                      ,                  xe2x80x83                ⁢        and                            (        4        )                                                      P                          patient              ⁡                              (                exp                )                                              =                                    P              base                        +                                          Gain                exp                            *              Flow                                      ,                            (        5        )            
where Ppatient(insp) is the pressured delivered to the patient during inhalation, Gaininsp is the constant used during inspiration to boost pressure based on the flow rate, Ppatient(exp) is the pressure delivered during exhalation, and Gainexp is the constant used during exhalation to reduce pressure based on the flow rate.
As with PAV, the PPAP parameters, i.e., Pbase , Gaininsp, and Gainexp, of the PPAP relation are set by the caregiver and do not change unless altered by the caregiver. For example, the caregiver may deem it appropriate to change the Gaininsp or Gainexp to maximize the comfort of the PPAP ventilation provided to the patient. It is also known to automate the process for setting the PPAP parameters automatically, automate the process for modifying these parameters, or to automate both the initial setting and modification of these parameters.
Regardless of whether the patient is provided with PAV or PPAP ventilation, conventional ventilation techniques involve starting the patient on either pressure support therapy by setting the various parameters and then starting the therapy. The patient is left to cope with the relatively sudden change in ventilation assistance once the ventilation assistance begins. Depending on the amount of assistance provided by the PAV or PPAP mode, this transition can be relatively large. Of course, the caregiver can manually adjust the parameters to provide a more comfortable transition for the patient. However, this requires the caregiver continually monitor and manually change the ventilator settings, which is time consuming. Likewise, when a patient is being weaned off of a ventilator, unless the caregiver manually, incrementally decreases the ventilation assistance provided by the PAV or PPAP mode of ventilation, the patient is forced to cope with the relative abrupt transition from receiving ventilation assisted breathing to receiving no such assistance.
Accordingly, it is an object of the present invention to provide a patient treatment system that overcomes the shortcomings of conventional patient treatment systems employing the PAV or PPAP ventilation mode This object is achieved according to one embodiment of the present invention by providing a patient treatment system that includes a gas flow generating system, a pressure controller, and a flow and volume sensing system. The gas flow generating system provides a flow of breathing gas to an airway of a patient. The pressure controller controls the pressure of breathing gas provided to the patient. The sensing system for detects the instantaneous flow rate {dot over (V)} and instantaneous volume V of breathing gas provided to the patient. The patient treatment system also includes a control unit that controls the operation of the pressure controller based on the output from the sensing system. More specifically, the control unit causes the pressure of breathing gas to be provided to the patient in accordance with the following relation:
Ppatient=R1(t){[R2(t)*(% Set)]*[((R3(t)*(FA)({dot over (V)}))+(R4(t)*(VA)(V))]+R5(t)*EPAP},
where Ppatient is the pressure at an airway of the patient, % Set is a multiplier, FA is a flow assist multiplier, VA is a volume assist multiplier, EPAP is an expiratory positive airway pressure, and R1(t), R2(t), R3(t), R4(t), and R5(t) are time-based parameter variation functions. In addition, at least one of the parameter variation functions R1(t), R2(t), R3(t), R4(t), and R5(t) is a function other than a unity function or one (1). By providing these parameter variation functions in the calculation of the pressure to be provided to the patient, the present invention enables one or more of the parameters in the above PAV equation to change over time, rather than subject the patient to an abrupt ventilation change.
In another embodiment of the present invention, the control unit causes the flow of breathing gas to be provided to the patient at a pressure in accordance with a following relation at least during a portion of the breathing cycle:
Ppatient=R1(t)[R2(t)Gain*{dot over (V)}+R3(t)Pbase],
where Ppatient is the pressure at an airway of such a patient, Gain is a constant, Pbase is a base-line pressure, and R1(t), R2(t), and R3(t) are time-based parameter variation functions, where at least one parameter variation function R1(t), R2(t), and R3(t) is a function other than a unity function or one (1). As in the embodiment discussed above, by providing these parameter variation functions in the calculation of the pressure to be provided to the patient, the present invention enables one or more of the parameters in the above PPAP equation to change over time, rather than subject the patient to an abrupt ventilation change. As described in greater detail below, the present invention also contemplates providing different PPAP equations, with the potential for different parameter variation functions, during the inspiratory and expiratory phases of the breathing cycle, as well as providing PPAP in combination with providing a continuous positive airway pressure (CPAP) or a bi-level positive airway pressure.
It is yet another object of the present invention to provide a method of treating a patient with PAV or PPAP mode of ventilation that does not suffer from the disadvantages associated with conventional PAV or PPAP ventilation techniques. This object is achieved by providing a method that includes providing a flow of breathing gas to an airway of a patient, detecting an instantaneous flow rate {dot over (V)} and an instantaneous volume V of the flow of breathing gas, and controlling a pressure of breathing gas provided to an airway of the patient in accordance with the following relation:
Ppatient=R1(t){[R2(t)*(% Set)]*[((R3(t)*(FA)({dot over (V)}))+(R4(t)*(VA)(V))]+R5(t)*EPAP},
where Ppatient is the pressure at an airway of the patient, % Set is a multiplier, FA is a flow assist multiplier, VA is a volume assist multiplier, EPAP is an expiratory positive airway pressure, and R1(t), R2(t), R3(t), R4(t), and R5(t) are time-based parameter variation functions, where at least one parameter variation function R1(t), R2(t), R3(t), R4(t), and R5(t) is a function other than a unity function or one (1).
Still further embodiments of the present invention contemplate using a similar method to provide the PPAP mode of ventilation to the patient, with the parameter variation functions being provided for one or more terms in the PPAP pressure calculation or calculations, depending on whether different PPAP equations are used for inspiration and expiration. As noted above, by providing parameter variation functions in the calculation of the pressure to be provided to the patient, the present invention enables one or more of the parameters in the PAV or PPAP pressure calculation equation to change over time, rather than subject the patient to an abrupt ventilation change.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.